(1) Field of the Invention
The present invention relates to an apparatus for producing a superconducting oxide film by metal organic chemical vapor deposition (MOCVD) and more particularly to an apparatus having functions to measure and control amounts of raw material gases, suitable for the production of a superconducting oxide film of large area or large length.
(2) Prior Art
A process for forming a superconducting oxide film by MOCVD comprises mixing vaporized source gases in a desired metal composition ratio and supplying the resulting mixture onto a substrate heated at a higher temperature than the decomposition temperatures of the source materials in an oxygen-containing atmosphere under reduced pressure, thereby depositing a desired film onto the substrate.
Generally, source materials for use in the MOCVD process for producing a superconducting oxide film are in a solid state at the ordinary temperature. In the actual formation of the film, solid powders of the source materials are heated at a temperature of about 100.degree. to about 200.degree. C. under reduced pressure and the generated sublimation gas is conveyed with an inert carrier gas into a film formation chamber, in which a substrate is placed. Among the source materials, particularly source materials of alkaline earth metals are thermally unstable and gradually decomposed in evaporator-containers to lose their sublimation properties. Thus, the amounts of raw material gases to be supplied to the film formation chamber change with time. In case of a superconductor of Y system represented, for example, by the following formula [1]: EQU Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. [ 1]
the metal composition ratio of Y:Ba:Cu=1:2:3 by atom changes and the proper superconducting characteristics is no more obtainable.
To solve the problem, it is possible to directly measure the amounts of raw material gases to be supplied, and feed back the measured values to raw material-heating temperatures or amounts of carrier gas, thereby controlling amounts of raw material gases to be generated. Several procedures have been so far proposed for measuring the amounts of raw material gases [Jpn. J. Appl. Phys., Vol. 29 (1990) L1072 and 1932]. In a procedure using a mass spectrometer as a detector, disclosed in Jpn. J. Appl. Phys., Vol. 29 (1990) L1072, it is necessary to introduce a portion of raw material gases from the raw material gas transfer line or the film formation chamber into the mass spectrometer. The raw material gases generated usually at a heating temperature of 100.degree. to 240.degree. C. will be solidified again at a lower temperature than the heating temperature or decomposed when the heating temperature is too high. Thus, the sampled raw material gases must be led to the analyzing section of the mass spectrometer while keeping the sampled gases hot carefully. However, it is difficult to heat the mass spectrometer itself, and thus it is difficult to obtain satisfactory quantitative results.
In a spectroscopic procedure disclosed in Jpn. J. Appl. Phys., Vol. 29 (1990) 1932, on the other hand, it is possible to directly measure the amounts of raw material gases in the film formation chamber, but it is indispensable for the measurement to form a plasma. In other words, the disclosed spectroscopic procedure is effective only for very limited processes based on the so called "plasma-assisted CVD". No quantitative correlation between the light intensities from the individual raw material gases and their concentrations are disclosed therein.